1. Field of the Invention
The present invention relates to a wheel grip factor estimation apparatus, particularly relates to an apparatus for estimating a grip factor indicative of a grip level of tire on a road surface in a lateral direction of a vehicle wheel.
2. Description of the Related Arts
In order to maintain a stability of a vehicle, there is known heretofore an apparatus for controlling a braking force applied to each wheel on the basis of vehicle state variable detected and determined, as disclosed in Japanese Patent Laid-open Publication No.6-99800, for example. In this publication, a target value of yaw rate is provided on the basis of a vehicle speed and a steering angle, and an over steering or an under steering is determined by a derived function of a difference between the actual value and the target value of the yaw rate. In case of the over steering, a braking slip is increased on a front wheel located outside of a curve when cornering, i.e., a braking force is increased on the front wheel located outside of the curve. Whereas, in case of the under steering, the braking slip is increased on the front wheel located inside of the curve. And, there is disclosed in Japanese Patent Laid-open Publication No.62-146754, an apparatus for setting a front wheel speed difference and a target value of lateral acceleration or yaw rate, on the basis of a steering angle and vehicle speed, to control brake and/or engine outputs.
In Japanese Patent Laid-open Publication No.11-99956, there is disclosed a steering apparatus for a vehicle with a variable steering angle ratio, to prevent wheels from being steered excessively, wherein an index named as a side force utilization ratio or lateral G utilization ratio is used. According to the apparatus as disclosed in the publication, a road coefficient of friction μ is estimated, to provide the side force utilization ratio. It is described that reaction force of a rack axis with the same steering angle applied by a road surface will be reduced in accordance with the road coefficient of friction μ, because the lower the road coefficient of friction μ is, the more a cornering power Cp of tire will be reduced. Therefore, it is concluded that the road coefficient of friction μ can be estimated by measuring the steering angle of front wheels and the reaction force of the rack axis, and comparing the reaction force of the rack axis against the steering angle of front wheels and a reference reaction force of the rack axis which is provided in advance as an inside model. Moreover, an equivalent friction circle is provided on the basis of the road coefficient of friction μ, then an amount of friction force used by a longitudinal force is subtracted from it to provide a maximal side force to be produced, and a ratio of the presently produced side force and the maximal side force is set as the side force utilization ratio. Or, a lateral G sensor may be provided for setting the lateral G utilization ratio on the basis of the lateral G detected by the sensor.
In the case where a vehicle has reached a limit for friction between road surface and tire, to cause an excessive under steering condition, it is required not only to control a yawing motion of the vehicle, i.e., a position of the vehicle on the road surface, but also to reduce the vehicle speed, in order to maintain a radius of cornering curve of the vehicle as intended by the vehicle driver. According to the apparatus as disclosed in the Publication No.6-99800, however, the vehicle behavior is determined after the tire reached the friction limit. When the vehicle speed is reduced in that situation, therefore, the cornering force will be reduced, whereby the tendency of under steering might be increased. Furthermore, according to the actual control system, as there is provided a dead zone for a control, the control generally begins after a certain vehicle behavior occurred.
As the curve of a vehicle road is formed into a clothoid curve, when the vehicle driver intends to trace the curve of the road, the steering wheel will be rotated with a gradually increasing amount. In the case where the vehicle speed is high when the vehicle has entered into the curve, therefore, the side force produced on the wheel will not balance with a centrifugal force, whereby the vehicle tends to be forced outside of the curve. In those cases, the apparatuses as disclosed in the Publication No.6-99800 and 62-146754 will operate to control the motion of the vehicle. However, as the controls begin at the cornering limit, the vehicle speed may not be reduced sufficiently by those controls. Therefore, it might be caused that the vehicle can not be prevented only by those controls from being forced outside of the curve.
With respect to the apparatus for estimating the road coefficient of friction μ, further publications have been known, such as Japanese Patent Laid-open Publication Nos.11-287749 and 6-221968. In the former, there is disclosed an apparatus for obtaining a characteristic of variation of steering torque to variation of steering angle, and estimating the road coefficient of friction μ on the basis of that characteristic. In the latter, there is disclosed an apparatus for detecting the road coefficient of friction μ on the basis of a relationship between a restoring moment of a wheel and a cornering force, with a hysteresis being reduced, to detect the road coefficient of friction μ before the wheel reaches a grip limit.
In the mean time, it is disclosed in AUTOMOTIVE ENGINEERING HANDBOOK, First Volume, for BASIC & THEORY, issued on Feb. 1, 1990 by Society of Automotive Engineers of Japan, Inc., Pages 179 and 180, such a state that a tire rotates on a road, skidding at a slip angle α, as shown in a part of FIG. 2 of the present application. As indicated by broken lines in FIG. 2, a tread surface of the tire contacts a road surface at a front end of the contacting surface including Point (A) in FIG. 2, and moves with the tire advanced, being adhesive to the road surface up to Point (B). The tire begins to slip when a deformation force by a lateral shearing deformation has become equal to a friction force, and departs from the road surface at a rear end including Point (C). In this case, a side force Fy produced on the overall contacting surface equals to a product of a deformed area of the tread in its lateral direction (as indicated by a hutching area in FIG. 2) multiplied by its lateral elastic coefficient per unit area. As shown in FIG. 13, a point of application of force for the side force Fy is shifted rearward (leftward in FIG. 2) from a point (O) on the center line of the tire, by a distance (en) which is called as a pneumatic trail. Accordingly, a moment Fy·en becomes an aligning torque (Tsa), which acts in such a direction to reduce the slip angle α, and which may be called as a self-aligning torque.
Next will be explained the case where the tire is installed on a vehicle, with reference to FIG. 3 which simplified FIG. 2. With respect to steered wheels of a vehicle, in general, a caster angle is provided so that a steering wheel can be returned to its original position smoothly, to produce a caster trail (ec). Therefore, the tire contacts the road surface at a point (O′), so that the moment for forcing the steering wheel to be positioned on its original position becomes Fy·(en+ec). When a lateral grip force of the tire is reduced to enlarge the slip area, the lateral deformation of the tread will result in changing a shape of ABC in FIG. 3 into a shape of ADC. As a result, the point of application of force for the side force Fy will be shifted forward in the advancing direction of the vehicle, from Point (H) to Point (J). That is, the pneumatic trail (en) will be reduced. Therefore, even in the case where the same side force Fy acts on the tire, if the adhesive area is relatively large and the slip area is relatively small, i.e., the lateral grip force of the tire is relatively large, the pneumatic trail (en) will be relatively large, so that the aligning torque Tsa will be relatively large. On the contrary, if the lateral grip force of the tire is lessened, and the slip area is enlarged, then the pneumatic trail (en) will become relatively small, so that the aligning torque Tsa will be reduced.
As described above, by monitoring the variation of the pneumatic trail (en), the grip level of the tire in its lateral direction can be detected. And, the variation of the pneumatic trail (en) results in the aligning torque Tsa, on the basis of which can be estimated a grip factor indicative of a grip level of the tire in its lateral direction, with respect to a front wheel for example (hereinafter simply referred to as grip factor). With respect to the grip factor, it can be estimated on the basis of a margin of side force for road friction, as described later in detail.
In this respect, the grip factor is clearly distinguished from the side force utilization ratio, or lateral G utilization ratio as described in the Japanese Publication No.11-99956, wherein the maximal side force which can be produced on the road surface is obtained on the basis of the road coefficient of friction μ. And, this road coefficient of friction μ is estimated on the basis of a reliability of the cornering power Cp (value of the side force per the slip angle of one degree) on the road coefficient of friction μ. However, the cornering power Cp relies not only on the road coefficient of friction μ, but also a shape of the area of the road contacting the tire (its contacting length and width to the road), and elasticity of the tread rubber. For example, in the case where water exists on the tread surface, or the case where the elasticity of the tread rubber has been changed due to wear of the tire or its temperature change, the cornering power Cp will vary, even if the road coefficient of friction μ is constant. In the Japanese Publication No.11-99956, therefore, nothing has been considered about the characteristic of the tire which constitutes the wheel.